Tag Archives: Darius Rydahl

Two-Wire LIN networking with Atmel (Part 2)

In the first part of this series we took a closer look at the basics of LIN networking, the key parameters for a two-wire LIN (Atmel) solution and the details of a LIN Bus power supply. In the second part of this series, we’ll discuss various aspects of slave node current consumption, specifically, system clock frequency, sleep mode power management and LIN scheduling power management.

According to Atmel engineering rep Darius Rydahl, the system clock frequency of the microcontroller (MCU) has the most significant effect on the slave node current consumption. The slave node current consumption is directly proportional to the clock frequency, an effect illustrated in Figure 4. Clearly, one should attempt to use the lowest clock frequency that enables the application to meet functional design requirements.

linnetworkingfigure4

In terms of power management-sleep mode, the overall current consumption of the two-wire LIN slave node can be further reduced by duty-cycling between low and high current operating modes, e.g. power-down/normal mode for the microcontroller and silent/normal mode for the LIN transceiver in between LIN data frames (see figure 5).

linnetworkingfigure5

Atmel AVR microcontrollers provide various sleep modes, allowing the user to tailor power consumption to the application’s requirements. In the case of the two-wire LIN application, the power-down mode provides the greatest current reduction when used in conjunction with the silent mode of the LIN transceiver,” Rydahl explained.

“In this mode, all generated clocks are shut down, allowing operation of asynchronous modules only (external interrupts, USI and watchdog). To wake up the microcontroller from power-down, the LIN master must first generate a LIN wake-up request followed by a LIN frame header. This process is shown in figure 6.”

linnetworkingfigure6

Upon wake-up, the microcontroller enters the normal mode and switches the EN pin (LIN transceiver enable) to HIGH at the start of each newly received LIN wake-up/frame packet.

During LIN data frames, the slave node microcontroller remains in normal mode and is able to provide an immediate data response upon receipt of the sync-break and message ID. At the end of the LIN data frame, the slave node returns to the power-down mode. It should be noted that operating the device in this manner will significantly reduce the average current consumption of the slave node.

On the subject of power management – LIN scheduling, the time between LIN frames, also known as the schedule table period, and the duration of the LIN frame define the power duty cycle of the slave node. This duty cycle affects the average current consumption of the two-wire LIN slave node. A typical LIN network operating at 19.2kbaud with a single frame, 8-bit message response has an average frame length of 2.95ms each. Figure 7 shows the effect of varying the schedule table period while connected to a slave node that is power duty cycling between power-down/silent and normal modes under these conditions.

linnetworkingfigure7

Clearly, lengthening the schedule table period reduces the slave node’s average current consumption. However, this benefit is bounded by the power-down/silent mode current and offers minimal benefit for schedule periods greater than one second.

Interested in learning more Two-Wire LIN networking with Atmel? Part one of this series can be read here, while part three will be posted tomorrow.

Two-Wire LIN networking with Atmel (Part 1)

Current-gen vehicles are packed with hundreds of sensors used to monitor and display parameters such as temperature and pressure. In most instances, these sensors are remotely located within a vehicle far away from the host microcontroller (MCU) responsible for monitoring and processing the sensor data.

As such, these sensors typically do not directly connect to a network (such as CAN or LIN) due to the vehicle wiring overhead associated with connecting to the network. One such method for overcoming this wiring limitation is to convert the standard three-wire LIN network to a two-wire implementation where the LIN slave nodes harvest power directly from the LIN bus master communication wire, thereby eliminating the need for an individual battery supply wire to each slave node.

linnetworkfigure1

As Atmel engineering rep Darius Rydahl notes, a standard LIN bus consists of a master node and up to 15 slave nodes connected to a single network. The physical LIN network is a three-wire configuration consisting of power (vehicle battery), ground and the LIN bus communication line. A pull-up resistor, RLIN, typically 1kΩ, is required on the master’s LIN bus line. Under normal LIN bus operation, this pull-up resistor provides a voltage bias on the LIN bus line to the slave nodes on the LIN network. It does not power the LIN slave nodes, rather slave node power is derived from the battery input to the device, as shown in Figure 1.

“It is possible to use a non-standard LIN network architecture that simplifies to two wires. This approach relies on the harvesting of power by a connected slave node directly from the LIN bus line, thus eliminating the need for an independent slave node battery supply line (see figure 2),” Rydahl told Bits & Pieces. “With the battery supply line removed, all that is required to power the slave node is a blocking diode, VDS and buffer capacitor, CVS_S, large enough to sustain the slave node supply voltage during the transmission of LIN data packets, which periodically pulls the LIN signal to ground.”

In this series, Bits & Pieces will outline the implementation of this two-wire approach and identify the inherent system-level tradeoffs that must be considered to fully realize a functional two-wire LIN network.

According to Rydahl, the key to successfully implementing a two-wire LIN network centers around the power requirements of the connected slave node. Simply put, the slave node must be supplied with sufficient power to maintain communication at the minimum system operating voltage: typically 9V. If this condition cannot be met, it is unlikely that the two-wire LIN implementation will be a viable solution. Key parameters that affect the slave node’s performance in a two-wire implementation include LIN bus power supply, slave node current consumption, slave node buffer capacitance and LIN Bus data protocol.

linnetworkfigure2

“In terms of the LIN Bus power supply, the two-wire LIN network is limited by the power supplied from the master to the slave node over the LIN bus line. Meaning, the supply to the LIN slave in this configuration will be dictated by the LIN bus master pull-up resistor, RLIN (see figure 2),” Rydahl continued. “The slave node has a fixed minimum input voltage operating requirement of 5.5V (reference: the Atmel ATA6624 LIN transceiver). In order to meet this minimum operating voltage requirement, the load current drawn by the slave node must not cause the voltage drop across the LIN master pull-up resistor to increase to the point at which the input voltage to the slave node drops below 5.5V.”

As Rydahl points out, this is the minimum operating voltage threshold for slave node voltage regulator operation. Indeed, figure 3 shows the maximum load current available to the slave node at the minimum supply voltage of 5.5V at different LIN master pull-up resistances.

linnetworkfigure3

“The 1kΩ master pull-up resistor specified in the LIN standard specification cannot be used in the two-wire configuration. The resistor is too large and, as a result, is unable to properly source the slave node load,” he said. “As such, the pull-up resistor must be reduced in size to the smallest value possible without exceeding the current limitation specification of the LIN driver. In the case of the typical Atmel LIN transceiver, the ATA6624, the recommended minimum pullup resistor value is 220Ω. Resistances lower than this could result in excessive current flow through the LIN transceiver when the LIN bus is asserted low.”

Interested in learning more about Two-Wire LIN networking with Atmel? Be sure to check out part two of this series here.

7/13-cell applications with Atmel’s ATA6870 (Part I)

A standard (automotive) battery measurement system using Atmel’s ATA6870 is capable of measuring the voltage of up to 6 battery cells. Several of these ICs can be stacked in series to measure the voltage of up to 96 battery cells simultaneously. For the majority of applications, the “stacked” battery measurement IC approach is sufficient, as the number of cells measured in these applications is a multiple of three, four or six.

batterymanagementfigure1

“In some instances, such as an e-bike application, the cell count of the battery may be of an odd number: 7 or 13 cells,” Atmel engineering rep Darius Rydahl told Bits & Pieces. “With these applications, the use of multiple, stacked ATA6870 circuits combined with a standard microcontroller (MCU) may not be the most cost-effective solution for the end application.”

According to Rydahl, a more practical, lower cost implementation is to use one ATA6870 chip in conjunction with an Atmel battery management microcontroller.

“The standard implementation of an ATA6870 battery management system consists of at least one ATA6870 battery measurement IC (maximum sixteen, connected in series) plus a general-purpose MCU for control and data processing,” Rydahl continued. “As you can see in the image above (Figure 1), the MCU is powered by the lower ATA6870 IC’s on-board 3.3V voltage regulator (VDDHVM). Communication occurs via SPI where data is transferred serially between multiple ATA6870 circuits, one IC to the next, to/from the MCU.”

As shown in Figure 1, a common ground reference is shared between the bottom ATA6870 device and the MCU. In this instance, there is no voltage offset between the MCU and the ATA6870 circuit, neatly eliminating the need for additional interface circuitry between the CLK and SPI pins of the two ICs.

batterymanagementfigure2

In applications where the total cell count is a multiple of 7 or 13, the designer can simply add additional ATA6870 ICs to the battery stack as shown in Figure 2. However, the 7 battery cells must be split between the ICs to maintain the minimum operating voltage of 6.7V for each ATA6870 IC.

“Atmel offers two possible solutions for the seven-cell application using a battery measurement MCU as shown in Figure 3. In this example, the ATA68670 IC can be paired with either the  ATmega32HVE2, or ATmega32HVB MCU,” said Rydahl.

batterymanagementfigure3

“Both MCUs have battery voltage and current measurement capabilities. The feature sets and peripheral offerings (number of cell measurement inputs, LIN bus interface, etc) of two MCUs are slightly different, so the specific requirements of the end application must be taken into consideration before selecting the MCU.”

Interested in learning more about using 7/13-cell applications with Atmel’s ATA6870? Be sure to check back tomorrow for part two of this series.